2. screen chamber

purpose of screening, types of screening, Design of screen chamber

1. by Prof V. V. Sasane (Assistant Professor) Department of Civil Engineering Sanjivani College of Engineering, Kopargaon , 423603 Subject: Environmental Engineering-II B.E. Civil Unit-II: Sewage Treatment 2.2 Screen Chamber Sanjivani Rural Education Society’s Sanjivani College of Engineering, Kopargaon , 423603 An Autonomous Institute Affiliated to Savitribai Phule Pune University, Pune

2. Screen Chamber Screening is the first unit operation in a wastewater treatment plant. A screen is a device with openings, generally of uniform size, used to retain suspended or floating coarse materials (pieces of cloths, wood, leaves, plastics, rags, papers etc.)found in the influent wastewater. These materials must be removed at the beginning of treatment process, otherwise could damage subsequent process equipment e.g. pumps, valves, pipe lines, impellers, thereby reducing overall treatment process reliability & effectiveness.

3. Objectives of Screening: To reduce the load on the subsequent treatment units. To reduce the damage to pumps and to protect the valves and the gates to ensure better operation. To remove any bulkey and unsighty floating matter carried in raw sewage. Features of the Screen: The screening element may consist any of the following : Parallel bars or rods, Gratings Wire meshes Perforated plates The openings are usually rectangular or circular, however could be of any shape . Depending on the size of opening, the screens may be coarse, medium or fine .

4. Types of Screens COARSE SCREENS Coarse screens have clear openings ranging from 6 to 150 mm (0.25 t0 6 in). Coarse screens consist of parallel bars, rods or wires, wire mesh or perforated plates with openings generally of circular or rectangular shapes. It is also called a “bar rack” and used to remove coarse solids such as rags and large objects that may clog or cause damage to other appurtenances. Based on the wastewater screening method used to clean them, coarse screens are classified into two types: 1. Hand cleaned coarse screens Used in the screening process in wastewater treatment at small facilities, hand cleaned coarse screens are hand raked. They are ideal to use as a standby during periods of high flow, or when more modern mechanical screening methods are under repair or maintenance. 2. Mechanically cleaned screens Mec hanically cleaned coarse screens increase efficiency and reduce problems in the wastewater treatment process. 

5. Fine Screens Fine screens are mechanically cleaned devices using perforated plates, woven wire cloth or closely spaced bars with clear openings of typically less than 6 mm. Fine screens are generally used for pre‐treatment of industrial wastes to remove materials which tend to produce excessive scum or foam . Fine screens are not normally suitable for sewage because of the clogging possibilities . Fine screens may be of the drum or disc type, mechanically cleaned and continuously operated .

6. Hydraulics of Screens Screens are provided to remove the materials which would impede the flow in the treatment plant. Hence, continuous cleaning arrangement can keep the interference of the solid materials to a minimum. On the other hand, periodic cleaning arrangements may cause surges of high flow after cleaning. Usually, the base of the screen is placed a few centimeters below the invert of the base channel and the grade of the influent conduit is steepened immediately preceding the screen.

7. Velocity through the Screens The velocity through the screen should be such that maximum amount of screenings (preferably 100%) are retained without undue deposition. Velocities of 0.6 to 1.2 m/s through the open area for the peak flows have been used satisfactorily. The velocity should not be less than 0.3 m/s to prevent the deposition of solids. A velocity of 0.8 m/s is appropriate for considerable amounts of storm water while preventing grit decomposition at the bottom of the screen. A straight channel succeeding the screen assures good velocity distribution across the screen and maximum effectiveness of the device.

8. 1. Head Loss through the Screens Head loss depends on the quantity and nature of screenings accumulated. Head loss can be calculated using the following formula : where : h= The head loss in meters. V = The velocity through the screens in meters/second. v = The velocity before the screen in meters/second. The value of h is usually 0.15m and should not exceed 0.3m for clogged hand screens.  

9. 2. Head Loss through the Screens Another approach to determine head loss through a bar rack is by Kirschmer’s equation: h β (W/b) 4/3 + h ν sin θ where: h = is the head loss in meters. β = the bar shape factor. W = is the maximum width of bar facing the flow in meters b= the minimum clear spacing between bars in m h ν the velocity head of flow approaching rack in m and is calculated as: h ν = (v 2 /2g) and θ = the angle of inclination of rack with the horizontal   Types of bars/screens Values Of β Sharp edged rectangular bar 2.42 Rectangular bar with semicircle upstream 1.83 Circular bar 1.79 Rectangular bar with both u/s and d/s face as semi‐circular 1.67

10. Disposal of Screening The quantity of screening varies depending on the type of screen as well as sewer system and its characteristics. Quantity of screening removed by bar screen mostly ranges between 0.0035 to 0.0375 / 1000 of wastewater treated, with an approximate typical value of 0.015 /1000 of wastewater. Screenings is usually disposed off along with municipal solid waste on sanitary landfill. It can also be brought back to the wastewater after passing it through grinders or disintegrator pumps. Other options include: Incineration (for large sewage treatment plant) or burring at the plant site (for smaller plants).  

11. THANKS THANKS Animation https ://www.youtube.com/watch?v=FMWNMNrfzC4 Part-2: https ://www.youtube.com/watch?v=SwjG_PDQ6NU Part -1: https ://www.youtube.com/watch?v=-9Z-ID8VMWE